On Thu, Mar 23, 2017 at 04:16:56PM +0100, Richard Biener wrote:
> On Thu, Mar 23, 2017 at 3:58 PM, Segher Boessenkool
> <seg...@kernel.crashing.org> wrote:
> > The algorithm fwprop uses never reconsiders a possible propagation,
> > although it could succeed if the def (in the def-use to propagate)
> > has changed. This causes fwprop to do infinite propagations, like
> > in the scenario in the PR, where we end up with effectively
> > B = A
> > A = B
> > D = A
> > where only propagations into the last statement are still tried, and
> > that loops (it becomes D = B, then back to D = A, etc.)
> >
> > Fixing this properly isn't easy; this patch instead limits the number
> > of propagations performed to the number of uses we originally had,
> > which is the maximum number of propagations that can be done if there
> > are no such infinite loops.
> >
> > Bootstrapped and regression checked on powerpc64-linux {-m64,-m32};
> > is this okay for trunk?
>
> https://gcc.gnu.org/ml/gcc-patches/2017-02/msg01485.html
>
> ?
What about it? That suggestion would make fwprop do *less* useful work,
while in principle the problem is that it *already* does not enough!
If fwprop actually tried all propagations (and never tried substituting
X with X, which it currently does), there would be no problem.
This patch is a workaround; it makes no difference on pretty much any
code, except this single testcase (which has undefined behaviour,
uninitialised variables).
Segher
